In a metal purifying process, additives have to be mixed with metal as uniformly as possible for the purpose of achieving significant improvements in intensity and quality, and molten metal has to be sufficiently stirred to achieve the purpose. Stirring of such molten metal is necessary also in various other fields of metal manufacture, such as manufacture of metallic particle dispersed composite materials, manufacture of ultra-clean metal materials by exhaustive separation of in-metal inclusions, and manufacture of high purity metal materials by utilizing a high level refining function, as well as manufacture of alloy, in particular, in a case of uniformly mixing alloy components having significantly different densities.
Hitherto, as techniques for performing contactless, intensive, and uniform stirring of molten metal, various types of electromagnetic stirring apparatuses using electromagnetic force have been proposed. It is known that, in the stirring of molten metal, it is effective to perform not only circumferential stirring but also vertical stirring. However, electromagnetic stirring using rotationally (circumferentially) traveling magnetic fields has drawbacks. When this technique is applied to stirring of molten metal in a vessel, a liquid surface is largely distorted due to rotation, so that a large amount of power cannot be input. In addition, only with rotary motion, the molten metal exhibits behavior like rigid body rotation, so that mixing of the molten metal is insufficient. Further, from the experiment results, it has been disclosed that, in comparison to the rotary motion of the molten metal, the motion in an axial direction (vertical direction) has a large resistance, so that sufficient effects cannot be obtained only from rotary motion by a rotary magnetic field.
In recent years, electromagnetic stirring apparatuses have been proposed in which the electromagnetic force is utilized to thereby simultaneously perform not only vertical stirring but also circumferential stirring. As a stirring apparatus simultaneously performing vertical and circumferential stirring by using electromagnetic force, there has been proposed a stirring apparatus of a type (Patent Document 1) of using two types coils, namely three-phase alternating current coils. The coils respectively generate a vertically traveling magnetic field and circumferentially traveling magnetic field, and are disposed exteriorly of a vessel, in which the induction effect is utilized to generate vertical and circumferential electromagnetic force, thereby to simultaneously perform vertical and circumferential stirring.
In addition, there has been an induction-type electromagnetic driving device (Patent Document 2). The driving device has a configuration in which a coil (“rotary coil”, hereinafter) for providing a rotary magnetic field to molten metal in a vessel is diagonally disposed in a spiral form about an iron core with respect to the axis of the vessel, a distortional magnetic field is applied by energization of the three-phase alternating current, thereby to provide an axially traveling magnetic field (traveling magnetic field) simultaneously with the rotary magnetic field.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-220323    [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-152600